QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 257 



either because a lesser stimulus is acting or because a particular 

 larva is less sensitive, it may become linked with the side-to-side 

 movements of normal locomotion. Here the reflex contraction 

 takes place just as before, but instead of being a separate, con- 

 spicuous wig-wag motion, it appears as a contraction added to 

 that involved in the side-to-side swinging of the head in normal 

 locomotion. This added contraction appears, of course, on the 

 side away from the light and its effect is to produce a gradual 

 turning away from the light until orientation is attained. In 

 l)oth cases when the anterior end is stimulated by a change of 

 intensity, it swings in the opposite direction to that in which it 

 was when the stimulus was received. 



These two methods by which the blowfly larva orients to a 

 single light are of such a nature that certain authors have called 

 the process, orientation by 'trial and error.' Undoubtedly if we 

 free the term from its original association with primitive intelli- 

 gence, as they have attempted to do, it characterizes certain 

 features of the orientation. But there are phenomena involved 

 in the orientation of the blowfly larva to lights from two sources 

 for which this interpretation is not adequate. The movements 

 of a bilaterally illuminated head involve a distribution of the 

 stimulus on the sensitive surfaces entirely different from that 

 produced by the movements of the head when there is but a 

 single source of light. A movement of the anterior end toward a 

 single light produces a sudden increase in light intensity on the 

 sensitive surfaces which is followed by a 'motor reflex,' throwing 

 the head back into the shade of the body again. Motion toward 

 the light is automatically prevented by the intervention of this 

 reflex. 



But with two lights of equal intensity acting on the larva from 

 opposite sides, a swaying of the anterior end will encounter no 

 changes of light intensity.^ Under these conditions there is, 



* The sources of light in these experiments were at such a distance from the 

 observation stage that the change of 2 or 3 mm. from the central point, which the 

 trial movements involved, would produce less than 0.3 per cent difference in the 

 opposing lights. The minimum difference at which a definite change in response 

 was detected was about 5 per cent. 



